1. Field of the Invention
The present invention relates to an electrode used in high pressure metal vapor discharge lamps such as a high pressure mercury vapor discharge lamp in which mercury and a rare gas are filled; a high pressure sodium vapor discharge lamp in which mercury, a rare gas and sodium are filled; a metal vapor discharge lamp in which mercury, a rare gas and a metal halide are filled or low pressure discharge lamps such as a fluorescent lamp. More particularly, it relates to an improvement of an electron emission material on the electrode.
2. Description of the Prior Arts
In a high pressure metal vapor discharge lamp such as a high pressure mercury vapor discharge lamp, a mixture of barium, strontium, calcium tungstate (Ba.sub.2-x Sr.sub.x CaWO.sub.6 : x is 0 to 0.5), beryllium oxide (BeO) and yttrium oxide (Y.sub.2 O.sub.3) has been used as an electron emission material coated on the electrode. In the electron emission material, a solid solution of BeO and Y.sub.2 O.sub.3 is formed whereby a lumen maintenance and a starting characteristic are advantageously excellent and stable during a long time operation of the lamps. However, BeO contained in the electron emission material is toxic to human body and the handling of the electron emission material should be carefully considered and the improvement of productivity is disadvantageously prevented. Moreover, in a high pressure metal vapor discharge lamp using the electron emission material, a phenomenon of the formation of black deposit is often found in the initial period. The black deposit is gradually changed to be white during the operation and it does not affect to the life characteristic of the lamp. However, it causes slightly low luminous flux value in the initial lightening period.
In a metal halide lamp, a rare earth metal oxide has been usually used as the electron emission material. Yttrium oxide is especially effective since it has excellent electron emitting characteristic. However, yttrium oxide has high melting point whereby the adhesion of the component on the electrode substrate is not satisfactorily attained to peel off the electron emission material during the operation of the discharge lamp. This causes deterioration of the activity of electron emission and the black coloring of the arc tube of the lamp is formed to cause a short life.
In a fluorescent lamp, an electron emission material of a mixture of barium, strontium, calcium carbonate and zirconium oxide is coated on a coil made of tungsten and the carbonate is thermally decomposed into oxides such as barium oxide in the step of preparing electrodes of a fluorescent lamp. The conventional electron emission material is reduced by tungsten as the electrode substrate during the operation of the discharge lamp and barium oxide (BaO) is converted into barium metal (Ba) by the following reaction. EQU 6BaO+W.fwdarw.Ba.sub.3 WO.sub.6 +3Ba.
The barium metal contributes to the electron emission from the electrode, however, a part of the barium metal is sputtered in the arc space and is adhered on the inner wall of the arc tube. A black coloring may be caused at the end of the arc tube when the quantity of the barium metal is much. Zirconium oxide is incorporated to decrease the reduction by the electrode substrate and to decrease the sputtering of the barium metal. The end blacking of the tube can be decreased by the incorporation of zirconium oxide, however, it may cause deterioration of the activity of electron emission of the electrode. When such electrode is used in the fluorescent lamp, the starting voltage is disadvantageously high.